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    <p data-fill-with="logo"><a href="http://www.w3.org/"><img alt="W3C" height="48" src="styles/logos/W3C" width="72"></a></p>
    <h1 class="p-name no-ref allcaps" id="title">HTML 5.1</h1>
    <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">W3C Candidate Recommendation, <time class="dt-updated" datetime="2016-06-21">21 June 2016</time></span></h2>
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  <nav data-fill-with="table-of-contents" id="toc"><p class="prev_next"><a href="index.html#contents">Table of contents</a> — <a href="infrastructure.html#infrastructure"><span class="secno">2</span> <span class="content">Common infrastructure</span></a> →</p>
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory"><li>
     <a href="introduction.html#introduction"><span class="secno">1</span> <span class="content">Introduction</span></a>
     <ol class="toc">
      <li><a href="introduction.html#background"><span class="secno">1.1</span> <span class="content">Background</span></a>
      </li><li><a href="introduction.html#audience"><span class="secno">1.2</span> <span class="content">Audience</span></a>
      </li><li><a href="introduction.html#scope"><span class="secno">1.3</span> <span class="content">Scope</span></a>
      </li><li><a href="introduction.html#introduction-history"><span class="secno">1.4</span> <span class="content">History</span></a>
      </li><li>
       <a href="introduction.html#design-notes"><span class="secno">1.5</span> <span class="content">Design notes</span></a>
       <ol class="toc">
        <li><a href="introduction.html#serializability-of-script-execution"><span class="secno">1.5.1</span> <span class="content">Serializability of script execution</span></a>
        </li><li><a href="introduction.html#compliance-with-other-specifications"><span class="secno">1.5.2</span> <span class="content">Compliance with other specifications</span></a>
        </li><li><a href="introduction.html#design-notes-extensibility"><span class="secno">1.5.3</span> <span class="content">Extensibility</span></a>
       </li></ol>
      </li><li><a href="introduction.html#html-vs-xhtml"><span class="secno">1.6</span> <span class="content">HTML vs XHTML</span></a>
      </li><li>
       <a href="introduction.html#structure-of-this-specification"><span class="secno">1.7</span> <span class="content">Structure of this specification</span></a>
       <ol class="toc">
        <li><a href="introduction.html#how-to-read-this-specification"><span class="secno">1.7.1</span> <span class="content">How to read this specification</span></a>
        </li><li><a href="introduction.html#typographic-conventions"><span class="secno">1.7.2</span> <span class="content">Typographic conventions</span></a>
       </li></ol>
      </li><li><a href="introduction.html#fingerprint"><span class="secno">1.8</span> <span class="content">Privacy concerns</span></a>
      </li><li>
       <a href="introduction.html#a-quick-introduction-to-html"><span class="secno">1.9</span> <span class="content">A quick introduction to HTML</span></a>
       <ol class="toc">
        <li><a href="introduction.html#writing-secure-applications-with-html"><span class="secno">1.9.1</span> <span class="content">Writing secure applications with HTML</span></a>
        </li><li><a href="introduction.html#common-pitfalls-to-avoid-when-using-the-scripting-apis"><span class="secno">1.9.2</span> <span class="content"> Common pitfalls to avoid when using the scripting APIs </span></a>
        </li><li><a href="introduction.html#how-to-catch-mistakes-when-writing-html-validators-and-conformance-checkers"><span class="secno">1.9.3</span> <span class="content"> How to catch mistakes when writing HTML: validators and conformance checkers </span></a>
       </li></ol>
      </li><li>
       <a href="introduction.html#conformance-requirements-for-authors"><span class="secno">1.10</span> <span class="content">Conformance requirements for authors</span></a>
       <ol class="toc">
        <li><a href="introduction.html#presentational-markup"><span class="secno">1.10.1</span> <span class="content">Presentational markup</span></a>
        </li><li><a href="introduction.html#syntax-errors"><span class="secno">1.10.2</span> <span class="content">Syntax errors</span></a>
        </li><li><a href="introduction.html#restrictions-on-content-models-and-on-attribute-values"><span class="secno">1.10.3</span> <span class="content"> Restrictions on content models and on attribute values </span></a>
       </li></ol>
      </li><li><a href="introduction.html#suggested-reading"><span class="secno">1.11</span> <span class="content">Suggested reading</span></a>
     </li></ol>
    </li></ol>
  </nav><main><section>
    <h2 class="heading settled" data-level="1" id="introduction"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="introduction.html#introduction"></a></h2>
    <h3 class="heading settled" data-level="1.1" id="background"><span class="secno">1.1. </span><span class="content">Background</span><a class="self-link" href="introduction.html#background"></a></h3>
    <p><em>This section is non-normative.</em></p>
    <p>HTML is the World Wide Web’s core markup language. Originally, HTML was primarily designed as a
  language for semantically describing scientific documents. Its general design, however, has
  enabled it to be adapted, over the subsequent years, to describe a number of other types of
  documents and even applications.</p>
    <h3 class="heading settled" data-level="1.2" id="audience"><span class="secno">1.2. </span><span class="content">Audience</span><a class="self-link" href="introduction.html#audience"></a></h3>
    <p><em>This section is non-normative.</em></p>
    <p>This specification is intended for authors of documents and scripts that use the features defined
  in this specification, implementors of tools that operate on pages that use the
  features defined in this specification, and individuals wishing to establish the correctness of
  documents or implementations with respect to the requirements of this specification.</p>
    <p>This document is probably not suited to readers who do not already have at least a passing
  familiarity with Web technologies, as in places it sacrifices clarity for precision, and brevity
  for completeness. More approachable tutorials and authoring guides can provide a gentler
  introduction to the topic.</p>
    <p>In particular, familiarity with the basics of DOM is necessary for a complete understanding of
  some of the more technical parts of this specification. An understanding of Web IDL, HTTP, XML,
  Unicode, character encodings, JavaScript, and CSS will also be helpful in places but is not
  essential.</p>
    <h3 class="heading settled" data-level="1.3" id="scope"><span class="secno">1.3. </span><span class="content">Scope</span><a class="self-link" href="introduction.html#scope"></a></h3>
    <p><em>This section is non-normative.</em></p>
    <p>This specification is limited to providing a semantic-level markup language and associated
  semantic-level scripting APIs for authoring accessible pages on the Web ranging from static
  documents to dynamic applications.</p>
    <p>The scope of this specification does not include providing mechanisms for media-specific
  customization of presentation (although default rendering rules for Web browsers are included at
  the end of this specification, and several mechanisms for hooking into CSS are provided as part
  of the language).</p>
    <p>The scope of this specification is not to describe an entire operating system. In particular,
  hardware configuration software, image manipulation tools, and applications that users would be
  expected to use with high-end workstations on a daily basis are out of scope. In terms of
  applications, this specification is targeted specifically at applications that would be expected
  to be used by users on an occasional basis, or regularly but from disparate locations, with low
  CPU requirements. Examples of such applications include online purchasing systems, searching
  systems, games (especially multiplayer online games), public telephone books or address books,
  communications software (e-mail clients, instant messaging clients, discussion software),
  document editing software, etc.</p>
    <h3 class="heading settled" data-level="1.4" id="introduction-history"><span class="secno">1.4. </span><span class="content">History</span><a class="self-link" href="introduction.html#introduction-history"></a></h3>
    <p><em>This section is non-normative.</em></p>
    <p>For its first five years (1990-1995), HTML went through a number of revisions and experienced a
  number of extensions, primarily hosted first at CERN, and then at the IETF.</p>
    <p>With the creation of the W3C, HTML’s development changed venue again. A first abortive attempt at
  extending HTML in 1995 known as HTML 3.0 then made way to a more pragmatic approach known as
  HTML 3.2, which was completed in 1997. HTML 4.01 quickly followed later that same year.</p>
    <p>The following year, the W3C membership decided to stop evolving HTML and instead begin work on
  an XML-based equivalent, called XHTML. This effort started with a reformulation of HTML 4.01 in
  XML, known as XHTML 1.0, which added no new features except the new serialization, and which was
  completed in 2000. After XHTML 1.0, the W3C’s focus turned to making it easier for other working
  groups to extend XHTML, under the banner of XHTML Modularization. In parallel with this, the W3C
  also worked on a new language that was not compatible with the earlier HTML and XHTML languages,
  calling it XHTML 2.0.</p>
    <p>Around the time that HTML’s evolution was stopped in 1998, parts of the API for HTML developed
  by browser vendors were specified and published under the name DOM Level 1 (in 1998) and DOM
  Level 2 Core and DOM Level 2 HTML (starting in 2000 and culminating in 2003). These efforts then
  petered out, with some DOM Level 3 specifications published in 2004 but the working group being
  closed before all the Level 3 drafts were completed.</p>
    <p>In 2003, the publication of XForms, a technology which was positioned as the next generation of
  Web forms, sparked a renewed interest in evolving HTML itself, rather than finding replacements
  for it. This interest was borne from the realization that XML’s deployment as a Web technology was
  limited to entirely new technologies (like RSS and later Atom), rather than as a replacement for
  existing deployed technologies (like HTML).</p>
    <p>A proof of concept to show that it was possible to extend HTML 4.01’s forms to provide many of the
  features that XForms 1.0 introduced, without requiring browsers to implement rendering engines
  that were incompatible with existing HTML Web pages, was the first result of this renewed
  interest. At this early stage, while the draft was already publicly available, and input was
  already being solicited from all sources, the specification was only under Opera Software’s
  copyright.</p>
    <p>The idea that HTML’s evolution should be reopened was tested at a W3C workshop in 2004, where
  some of the principles that underlie the HTML work (described below), as well as the
  aforementioned early draft proposal covering just forms-related features, were presented to the
  W3C jointly by Mozilla and Opera. The proposal was rejected on the grounds that the proposal
  conflicted with the previously chosen direction for the Web’s evolution; the W3C staff and
  membership voted to continue developing XML-based replacements instead.</p>
    <p>Shortly thereafter, Apple, Mozilla, and Opera jointly announced their intent to continue working
  on the effort under the umbrella of a new venue called the WHATWG. A public mailing list was
  created, and the draft was moved to the WHATWG site. The copyright was subsequently amended to be
  jointly owned by all three vendors, and to allow reuse of the specification.</p>
    <p>The WHATWG was based on several core principles, in particular that technologies need to be
  backwards compatible, that specifications and implementations need to match even if this means
  changing the specification rather than the implementations, and that specifications need to be
  detailed enough that implementations can achieve complete interoperability without
  reverse-engineering each other.</p>
    <p>The latter requirement in particular required that the scope of the HTML specification include
  what had previously been specified in three separate documents: HTML 4.01, XHTML 1.1, and DOM
  Level 2 HTML. It also meant including significantly more detail than had previously been
  considered the norm.</p>
    <p>In 2006, the W3C indicated an interest to participate in the development of HTML 5.0 after all,
  and in 2007 formed a working group chartered to work with the WHATWG on the development of the
  HTML specification. Apple, Mozilla, and Opera allowed the W3C to publish the specification under
  the W3C copyright, while keeping a version with the less restrictive license on the WHATWG site.</p>
    <p>For a number of years, both groups then worked together under the same editor: Ian Hickson. In
  2011, the groups came to the conclusion that they had different goals: the W3C wanted to draw a
  line in the sand for features for a HTML 5.0 Recommendation, while the WHATWG wanted to continue
  working on a Living Standard for HTML, continuously maintaining the specification and adding new
  features. In mid 2012, a new editing team was introduced at the W3C to take care of creating a
  HTML 5.0 Recommendation and prepare a Working Draft for the next HTML version.</p>
    <p>Since then, the W3C HTML WG has been cherry picking patches from the WHATWG that resolved bugs
  registered on the W3C HTML specification or more accurately represented implemented reality in
  user agents. At time of publication of this document, patches from the <a data-link-type="dfn" href="https://whatwg.org/specs/web-apps/current-work/#">WHATWG HTML specification</a> have been merged until January 12, 2016. The W3C HTML editors
  have also added patches that resulted from discussions and decisions made by the W3C HTML WG as
  well a bug fixes from bugs not shared by the WHATWG.</p>
    <p>A separate document is published to document the differences between the HTML specified in this
  document and the language described in the HTML 4.01 specification. <a data-link-type="biblio" href="references.html#biblio-html5-diff">[HTML5-DIFF]</a></p>
    <h3 class="heading settled" data-level="1.5" id="design-notes"><span class="secno">1.5. </span><span class="content">Design notes</span><a class="self-link" href="introduction.html#design-notes"></a></h3>
    <p><em>This section is non-normative.</em></p>
    <p>It must be admitted that many aspects of HTML appear at first glance to be nonsensical and
  inconsistent.</p>
    <p>HTML, its supporting DOM APIs, as well as many of its supporting technologies, have been
  developed over a period of several decades by a wide array of people with different priorities
  who, in many cases, did not know of each other’s existence.</p>
    <p>Features have thus arisen from many sources, and have not always been designed in especially
  consistent ways. Furthermore, because of the unique characteristics of the Web, implementation
  bugs have often become de-facto, and now de-jure, standards, as content is often unintentionally
  written in ways that rely on them before they can be fixed.</p>
    <p>Despite all this, efforts have been made to adhere to certain design goals. These are described
  in the next few subsections.</p>
    <h4 class="heading settled" data-level="1.5.1" id="serializability-of-script-execution"><span class="secno">1.5.1. </span><span class="content">Serializability of script execution</span><a class="self-link" href="introduction.html#serializability-of-script-execution"></a></h4>
    <p><em>This section is non-normative.</em></p>
    <p>To avoid exposing Web authors to the complexities of multithreading, the HTML and DOM APIs are
  designed such that no script can ever detect the simultaneous execution of other scripts. Even
  with <code class="idl"><a data-link-type="idl" href="https://www.w3.org/TR/workers/#worker">workers</a></code>, the intent is that the behavior of implementations
  can be thought of as completely serializing the execution of all scripts in all <a data-link-type="dfn" href="browsers.html#browsing-context" id="ref-for-browsing-context-1">browsing contexts</a>.</p>
    <h4 class="heading settled" data-level="1.5.2" id="compliance-with-other-specifications"><span class="secno">1.5.2. </span><span class="content">Compliance with other specifications</span><a class="self-link" href="introduction.html#compliance-with-other-specifications"></a></h4>
    <p><em>This section is non-normative.</em></p>
    <p>This specification interacts with and relies on a wide variety of other specifications. In certain
  circumstances, unfortunately, conflicting needs have led to this specification violating the
  requirements of these other specifications. Whenever this has occurred, the transgressions have
  each been noted as a "<dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport="" id="willful-violation">willful violation</dfn>", and the reason for the violation has been
  noted.</p>
    <h4 class="heading settled" data-level="1.5.3" id="design-notes-extensibility"><span class="secno">1.5.3. </span><span class="content">Extensibility</span><a class="self-link" href="introduction.html#design-notes-extensibility"></a></h4>
    <p><em>This section is non-normative.</em></p>
    <p>HTML has a wide array of extensibility mechanisms that can be used for adding semantics in a safe
  manner:</p>
    <ul>
     <li data-md="">
      <p>Authors can use the <code><a data-link-type="element-sub" href="dom.html#element-attrdef-global-class" id="ref-for-element-attrdef-global-class-1">class</a></code> attribute to extend elements, effectively creating their
own elements, while using the most applicable existing "real" HTML element, so that browsers and
other tools that don’t know of the extension can still support it somewhat well. This is the
tack used by microformats, for example.</p>
     </li><li data-md="">
      <p>Authors can include data for inline client-side scripts or server-side site-wide scripts to
process using the <code><a data-link-type="element-sub" href="dom.html#element-attrdef-global-data" id="ref-for-element-attrdef-global-data-1">data-*=""</a></code> attributes. These are guaranteed to never be
touched by browsers, and allow scripts to include data on HTML elements that scripts can then
look for and process.</p>
     </li><li data-md="">
      <p>Authors can use the <code><a data-link-type="element" href="document-metadata.html#elementdef-meta" id="ref-for-elementdef-meta-1">meta name="" content=""</a></code> mechanism to
include page-wide metadata by registering <a data-link-type="dfn" href="document-metadata.html#register-the-names" id="ref-for-register-the-names-1">extensions to the predefined set of metadata names</a>.</p>
     </li><li data-md="">
      <p>Authors can use the <code><a data-link-type="element-sub" href="document-metadata.html#element-attrdef-link-rel" id="ref-for-element-attrdef-link-rel-1">rel=""</a></code> mechanism to annotate links with specific meanings by
registering <a data-link-type="dfn" href="links.html#link-types-extensions-to-the-predefined-set-of-link-type" id="ref-for-link-types-extensions-to-the-predefined-set-of-link-type-1">extensions to the predefined set of link types</a>. This is also used by
microformats.</p>
     </li><li data-md="">
      <p>Authors can embed raw data using the <code><a data-link-type="element" href="semantics-scripting.html#elementdef-script" id="ref-for-elementdef-script-1">script type=""</a></code> mechanism with a custom
type, for further handling by inline or server-side scripts.</p>
     </li><li data-md="">
      <p>Authors can create <a data-link-type="dfn" href="infrastructure.html#plugin" id="ref-for-plugin-1">plugins</a> and invoke them using the <code><a data-link-type="element" href="semantics-embedded-content.html#elementdef-embed" id="ref-for-elementdef-embed-1">embed</a></code> element. This is how
Flash works.</p>
     </li><li data-md="">
      <p>Authors can extend APIs using the JavaScript prototyping mechanism. This is widely used by
script libraries, for instance.</p>
    </li></ul>
    <h3 class="heading settled" data-level="1.6" id="html-vs-xhtml"><span class="secno">1.6. </span><span class="content">HTML vs XHTML</span><a class="self-link" href="introduction.html#html-vs-xhtml"></a></h3>
    <p><em>This section is non-normative.</em></p>
    <p>This specification defines an abstract language for describing documents and applications, and
  some APIs for interacting with in-memory representations of resources that use this language.</p>
    <p>The in-memory representation is known as "DOM HTML", or "the DOM" for short.</p>
    <p>There are various concrete syntaxes that can be used to transmit resources that use this abstract
  language, two of which are defined in this specification.</p>
    <p>The first such concrete syntax is the HTML syntax. This is the format suggested for most authors.
  It is compatible with most legacy Web browsers. If a document is transmitted with the <a href="iana.html#text-html"><code>text/html</code></a> <a data-link-type="dfn" href="infrastructure.html#mime-type" id="ref-for-mime-type-1">MIME type</a>, then it will be processed as an
  HTML document by Web browsers. This specification defines the latest version of the HTML syntax,
  known as "HTML 5.1".</p>
    <p>The second concrete syntax is the XHTML syntax, which is an application of XML. When a document
  is transmitted with an <a data-link-type="dfn" href="infrastructure.html#xml-mime-type" id="ref-for-xml-mime-type-1">XML MIME type</a>, such as <a href="iana.html#application-xhtmlxml"><code>application/xhtml+xml</code></a>, then it is treated as an
  XML document by Web browsers, to be parsed by an XML processor. Authors are reminded that the
  processing for XML and HTML differs; in particular, even minor syntax errors will prevent a
  document labeled as XML from being rendered fully, whereas they would be ignored in the HTML
  syntax. This specification defines the latest version of the XHTML syntax, known as "XHTML
  5.1".</p>
    <p>The DOM, the HTML syntax, and the XHTML syntax cannot all represent the same content. For
  example, namespaces cannot be represented using the HTML syntax, but they are supported in the
  DOM and in the XHTML syntax. Similarly, documents that use the <code><a data-link-type="element" href="semantics-scripting.html#elementdef-noscript" id="ref-for-elementdef-noscript-1">noscript</a></code> feature can
  be represented using the HTML syntax, but cannot be represented with the DOM or in the XHTML
  syntax. Comments that contain the string "<code>--&gt;</code>" can only be represented in the
  DOM, not in the HTML and XHTML syntaxes.</p>
    <h3 class="heading settled" data-level="1.7" id="structure-of-this-specification"><span class="secno">1.7. </span><span class="content">Structure of this specification</span><a class="self-link" href="introduction.html#structure-of-this-specification"></a></h3>
    <p><em>This section is non-normative.</em></p>
    <p>This specification is divided into the following major sections:</p>
    <div>
     <dl>
      <dt data-md="">
       <p><a href="introduction.html#introduction">§1 Introduction</a></p>
      </dt><dd data-md="">
       <p>Non-normative materials providing a context for the HTML standard.</p>
      </dd><dt data-md="">
       <p><a href="infrastructure.html#infrastructure">§2 Common infrastructure</a></p>
      </dt><dd data-md="">
       <p>The conformance classes, algorithms, definitions, and the common underpinnings of the rest of
the specification.</p>
      </dd><dt data-md="">
       <p><a href="dom.html#dom">§3 Semantics, structure, and APIs of HTML documents</a></p>
      </dt><dd data-md="">
       <p>Documents are built from elements. These elements form a tree using the DOM. This section
defines the features of this DOM, as well as introducing the features common to all
elements, and the concepts used in defining elements.</p>
      </dd><dt data-md="">
       <p><a href="semantics.html#semantics">§4 The elements of HTML</a></p>
      </dt><dd data-md="">
       <p>Each element has a predefined meaning, which is explained in this section. Rules for authors
on how to use the element, along with user agent requirements for how to
handle each element, are also given. This includes large signature features of HTML
such as video playback and subtitles, form controls and form submission, and a 2D graphics
API known as the HTML canvas.</p>
      </dd><dt data-md="">
       <p><a href="editing.html#editing">§5 User interaction</a></p>
      </dt><dd data-md="">
       <p>HTML documents can provide a number of mechanisms for users to interact with and modify
content, which are described in this section, such as how focus works, and drag-and-drop.</p>
      </dd><dt data-md="">
       <p><a href="browsers.html#browsers">§6 Loading Web pages</a></p>
      </dt><dd data-md="">
       <p>HTML documents do not exist in a vacuum — this section defines many of the features
that affect environments that deal with multiple pages, such as Web browsers and offline
caching of Web applications.</p>
      </dd><dt data-md="">
       <p><a href="webappapis.html#webappapis">§7 Web application APIs</a></p>
      </dt><dd data-md="">
       <p>This section introduces basic features for scripting of applications in HTML.</p>
      </dd><dt data-md="">
       <p><a href="syntax.html#syntax">§8 The HTML syntax</a></p>
      </dt><dt data-md="">
       <p><a href="xhtml.html#xhtml">§9 The XHTML syntax</a></p>
      </dt><dd data-md="">
       <p>All of these features would be for naught if they couldn’t be represented in a serialized
form and sent to other people, and so these sections define the syntaxes of HTML and
XHTML, along with rules for how to parse content using those syntaxes.</p>
      </dd><dt data-md="">
       <p><a href="rendering.html#rendering">§10 Rendering</a></p>
      </dt><dd data-md="">
       <p>This section defines the default rendering rules for Web browsers.</p>
     </dd></dl>
     <p>There are also some appendices, listing <a href="obsolete.html#obsolete">§11 Obsolete features</a> and <a href="iana.html#iana">§12 IANA considerations</a>, and several indices.</p>
    </div>
    <h4 class="heading settled" data-level="1.7.1" id="how-to-read-this-specification"><span class="secno">1.7.1. </span><span class="content">How to read this specification</span><a class="self-link" href="introduction.html#how-to-read-this-specification"></a></h4>
    <p>This specification should be read like all other specifications. First, it should be read
  cover-to-cover, multiple times. Then, it should be read backwards at least once. Then it should be
  read by picking random sections from the contents list and following all the cross-references.</p>
    <p>As described in the conformance requirements section below, this specification describes
  conformance criteria for a variety of conformance classes. In particular, there are conformance
  requirements that apply to <em>producers</em>, for example authors and the documents they create,
  and there are conformance requirements that apply to <em>consumers</em>, for example Web browsers.
  They can be distinguished by what they are requiring: a requirement on a producer states what is
  allowed, while a requirement on a consumer states how software is to act.</p>
    <div class="example" id="example-7e5c439b"><a class="self-link" href="introduction.html#example-7e5c439b"></a> For example, "the <code>foo</code> attribute’s value must be a <a data-link-type="dfn" href="infrastructure.html#valid-integer" id="ref-for-valid-integer-1">valid integer</a>" is a
    requirement on producers, as it lays out the allowed values; in contrast, the requirement "the <code>foo</code> attribute’s value must be parsed using the <a data-link-type="dfn" href="infrastructure.html#parse-token-as-an-integer" id="ref-for-parse-token-as-an-integer-1">rules for parsing integers</a>"
    is a requirement on consumers, as it describes how to process the content. </div>
    <p><strong>Requirements on producers have no bearing whatsoever on consumers.</strong></p>
    <div class="example" id="example-287546e8"><a class="self-link" href="introduction.html#example-287546e8"></a> Continuing the above example, a requirement stating that a particular attribute’s value is
    constrained to being a <a data-link-type="dfn" href="infrastructure.html#valid-integer" id="ref-for-valid-integer-2">valid integer</a> emphatically does <em>not</em> imply anything about
    the requirements on consumers. It might be that the consumers are in fact required to treat the
    attribute as an opaque string, completely unaffected by whether the value conforms to the
    requirements or not. It might be (as in the previous example) that the consumers are required to
    parse the value using specific rules that define how invalid (non-numeric in this case) values
    are to be processed. </div>
    <h4 class="heading settled" data-level="1.7.2" id="typographic-conventions"><span class="secno">1.7.2. </span><span class="content">Typographic conventions</span><a class="self-link" href="introduction.html#typographic-conventions"></a></h4>
    <div>
      This is a definition, requirement, or explanation. 
     <p class="note" role="note">This is a note.</p>
     <p class="example" id="example-263fb1aa"><a class="self-link" href="introduction.html#example-263fb1aa"></a>This is an example.</p>
     <p class="XXX">This is an open issue.</p>
     <p class="warning">This is a warning.</p>
<pre class="idl-ignore def" data-highlight="webidl" data-no-idl="">interface Example {
    // this is an IDL definition
};
</pre>
     <dl class="domintro">
      <dt><var>variable</var> = <var>object</var> . <code>method</code>( [ <var>optionalArgument</var> ] )
      </dt><dd>This is a note to authors describing the usage of an interface.
     </dd></dl>
<pre class="css highlight"><span class="c">/* this is a CSS fragment */</span>
</pre>
     <p>The defining instance of a term is marked up like <dfn class="dfn-paneled" data-dfn-for="conventions" data-dfn-type="dfn" data-noexport="" id="conventions-this">this</dfn>. Uses of that term
    are marked up like <a data-link-type="dfn" href="introduction.html#conventions-this" id="ref-for-conventions-this-1">this</a> or like <a data-link-type="dfn" href="introduction.html#conventions-this" id="ref-for-conventions-this-2"><i>this</i></a>.</p>
     <p>The defining instance of an element, attribute, or API is marked up like <dfn class="dfn-paneled" data-dfn-for="conventions" data-dfn-type="element" data-noexport="" id="elementdef-conventions-this"><code>this</code></dfn>. References to that element, attribute, or API are
    marked up like <code><a data-link-type="element" href="introduction.html#elementdef-conventions-this" id="ref-for-elementdef-conventions-this-1">this</a></code>.</p>
     <p>Other code fragments are marked up <code>like this</code>.</p>
     <p>Byte sequences with bytes in the range 0x00 to 0x7F, inclusive, are marked up like
    `<code>this</code>`.</p>
     <p>Variables are marked up like <var>this</var>.</p>
     <p>In an algorithm, steps in <a data-link-type="dfn" href="webappapis.html#synchronous-section" id="ref-for-synchronous-section-1">synchronous sections</a> are marked with ⌛.</p>
     <p>In some cases, requirements are given in the form of lists with conditions and corresponding
    requirements. In such cases, the requirements that apply to a condition are always the first set
    of requirements that follow the condition, even in the case of there being multiple sets of
    conditions for those requirements. Such cases are presented as follows:</p>
     <dl class="switch">
      <dt>This is a condition
      </dt><dt>This is another condition
      </dt><dd>This is the requirement that applies to the conditions above.
      </dd><dt>This is a third condition
      </dt><dd>This is the requirement that applies to the third condition.
     </dd></dl>
    </div>
    <h3 class="heading settled" data-level="1.8" id="fingerprint"><span class="secno">1.8. </span><span class="content">Privacy concerns</span><a class="self-link" href="introduction.html#fingerprint"></a></h3>
    <p><em>This section is non-normative.</em></p>
    <p>Some features of HTML trade user convenience for a measure of user privacy.</p>
    <p>In general, due to the Internet’s architecture, a user can be distinguished from another by the
  user’s IP address. IP addresses do not perfectly match to a user; as a user moves from device to
  device, or from network to network, their IP address will change; similarly, NAT routing, proxy
  servers, and shared computers enable packets that appear to all come from a single IP address to
  actually map to multiple users. Technologies such as onion routing can be used to further
  anonymize requests so that requests from a single user at one node on the Internet appear to come
  from many disparate parts of the network.</p>
    <p>However, the IP address used for a user’s requests is not the only mechanism by which a user’s
  requests could be related to each other.</p>
    <p>Cookies, for example, are designed specifically to enable this, and are the basis of most of the
  Web’s session features that enable you to log into a site with which you have an account.</p>
    <p>Application caches have similar implications with respect to privacy, for example if the
  site can identify the user when providing the cache, it can store data in the cache that can be
  used for cookie resurrection.</p>
    <p>There are other mechanisms that are more subtle. Certain characteristics of a user’s system can be
  used to distinguish groups of users from each other; by collecting enough such information, an
  individual user’s browser’s "digital fingerprint" can be computed, which can be as good, if not
  better, as an IP address in ascertaining which requests are from the same user.</p>
    <p>Grouping requests in this manner, especially across multiple sites, can be used for both benign
  (and even arguably positive) purposes, as well as for malevolent purposes. An example of a
  reasonably benign purpose would be determining whether a particular person seems to prefer sites
  with dog illustrations as opposed to sites with cat illustrations (based on how often they visit
  the sites in question) and then automatically using the preferred illustrations on subsequent
  visits to participating sites. Malevolent purposes, however, could include governments combining
  information such as the person’s home address (determined from the addresses they use when getting
  driving directions on one site) with their apparent political affiliations (determined by
  examining the forum sites that they participate in) to determine whether the person should be
  prevented from voting in an election.</p>
    <p>Since the malevolent purposes can be remarkably evil, user agent implementors are encouraged to
  consider how to provide their users with tools to minimize leaking information that could be used
  to fingerprint a user.</p>
    <p>Unfortunately, as the first paragraph in this section implies, sometimes there is great benefit
  to be derived from exposing the very information that can also be used for fingerprinting
  purposes, so it’s not as easy as simply blocking all possible leaks. For instance, the ability to
  log into a site to post under a specific identity requires that the user’s requests be
  identifiable as all being from the same user. More subtly, though, information such as how wide
  text is, which is necessary for many effects that involve drawing text onto a canvas (e.g., any
  effect that involves drawing a border around the text) also leaks information that can be used to
  group a user’s requests. (In this case, by potentially exposing, via a brute force search, which
  fonts a user has installed, information which can vary considerably from user to user.)</p>
    <p>Features in this specification which can be <dfn class="dfn-paneled" data-dfn-type="dfn" data-lt="for privacy" data-noexport="" id="fingerprinting-vector">used to fingerprint the user</dfn> are marked as
  this paragraph is. <a class="fingerprint" href="introduction.html#fingerprinting-vector" id="ref-for-fingerprinting-vector-1"><img alt="(This is a fingerprinting vector.)" height="21" src="images/fingerprint.png" width="15"></a></p>
    <p>Other features in the platform can be used for the same purpose, though, including, though not
  limited to:</p>
    <ul>
     <li data-md="">
      <p>The exact list of which features a user agents supports.</p>
     </li><li data-md="">
      <p>The maximum allowed stack depth for recursion in script.</p>
     </li><li data-md="">
      <p>Features that describe the user’s environment, like Media Queries and the <code class="idl"><a data-link-type="idl" href="https://www.w3.org/TR/cssom-view/#screen">Screen</a></code> object. <a data-link-type="biblio" href="references.html#biblio-mediaq">[MEDIAQ]</a> <a data-link-type="biblio" href="references.html#biblio-cssom-view">[CSSOM-VIEW]</a></p>
     </li><li data-md="">
      <p>The user’s time zone.</p>
    </li></ul>
    <h3 class="heading settled" data-level="1.9" id="a-quick-introduction-to-html"><span class="secno">1.9. </span><span class="content">A quick introduction to HTML</span><a class="self-link" href="introduction.html#a-quick-introduction-to-html"></a></h3>
    <p><em>This section is non-normative.</em></p>
    <p>A basic HTML document looks like this:</p>
<pre class="highlight" id="intro-early-example"><span class="cp">&lt;!DOCTYPE html&gt;</span>
<span class="nt">&lt;html&gt;</span>
  <span class="nt">&lt;head&gt;</span>
    <span class="nt">&lt;title&gt;</span>Sample page<span class="nt">&lt;/title&gt;</span>
  <span class="nt">&lt;/head&gt;</span>
  <span class="nt">&lt;body&gt;</span>
    <span class="nt">&lt;h1&gt;</span>Sample page<span class="nt">&lt;/h1&gt;</span>
    <span class="nt">&lt;p&gt;</span>This is a <span class="nt">&lt;a</span> <span class="na">href=</span><span class="s">"demo.html"</span><span class="nt">&gt;</span>simple<span class="nt">&lt;/a&gt;</span> sample.<span class="nt">&lt;/p&gt;</span>
    <span class="c">&lt;!-- this is a comment --&gt;</span>
  <span class="nt">&lt;/body&gt;</span>
<span class="nt">&lt;/html&gt;</span>
</pre>
    <p>HTML documents consist of a tree of elements and text. Each element is denoted in the source by
  a <a data-link-type="dfn" href="syntax.html#start-tag" id="ref-for-start-tag-1">start tag</a>, such as "<code><a data-link-type="element" href="sections.html#elementdef-body" id="ref-for-elementdef-body-1">&lt;body&gt;</a></code>", and an <a data-link-type="dfn" href="syntax.html#end-tag" id="ref-for-end-tag-1">end tag</a>, such as
  "<code><a data-link-type="element" href="sections.html#elementdef-body" id="ref-for-elementdef-body-2">&lt;/body&gt;</a></code>". (Certain start tags and end tags can in certain cases be <a data-link-type="dfn" href="syntax.html#optional-start-and-end-tags" id="ref-for-optional-start-and-end-tags-1">omitted</a> and are implied by other tags.)</p>
    <p>Tags have to be nested such that elements are all completely within each other, without
  overlapping:</p>
<pre class="bad highlight"><span class="nt">&lt;p&gt;</span>This is <span class="nt">&lt;em&gt;</span>very <span class="nt">&lt;strong&gt;</span>wrong<span class="nt">&lt;/em&gt;</span>!<span class="nt">&lt;/strong&gt;&lt;/p&gt;</span>
</pre>
<pre class="highlight"><span class="nt">&lt;p&gt;</span>This <span class="nt">&lt;em&gt;</span>is <span class="nt">&lt;strong&gt;</span>correct<span class="nt">&lt;/strong&gt;</span>.<span class="nt">&lt;/em&gt;&lt;/p&gt;</span>
</pre>
    <p>This specification defines a set of elements that can be used in HTML, along with rules about the
  ways in which the elements can be nested.</p>
    <p>Elements can have attributes, which control how the elements work. In the example below, there
  is a <a data-link-type="dfn" href="links.html#hyperlink" id="ref-for-hyperlink-1">hyperlink</a>, formed using the <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-a" id="ref-for-elementdef-a-1">a</a></code> element and its <code><a data-link-type="element-sub" href="links.html#element-attrdef-a-href" id="ref-for-element-attrdef-a-href-1">href</a></code> attribute:</p>
<pre class="highlight"><span class="nt">&lt;a</span> <span class="na">href=</span><span class="s">"demo.html"</span><span class="nt">&gt;</span>simple<span class="nt">&lt;/a&gt;</span>
</pre>
    <p><a data-link-type="dfn" href="syntax.html#attribute" id="ref-for-attribute-1">Attributes</a> are placed inside the start tag, and consist of a <a data-link-type="dfn" href="syntax.html#name" id="ref-for-name-1">name</a> and a <a data-link-type="dfn" href="syntax.html#attribute-value" id="ref-for-attribute-value-1">value</a>, separated by an
  "<code>=</code>" character. The attribute value can remain <a data-link-type="dfn" href="syntax.html#unquoted" id="ref-for-unquoted-1">unquoted</a> if it doesn’t contain <a data-link-type="dfn" href="infrastructure.html#space-characters" id="ref-for-space-characters-1">space characters</a> or any of <code>"</code> <code>'</code> <code>`</code> <code>=</code> <code>&lt;</code> or <code>&gt;</code>. Otherwise, it has to be quoted using either single or
  double quotes. The value, along with the "<code>=</code>" character, can be omitted altogether if
  the value is the empty string.</p>
<pre class="highlight"><span class="c">&lt;!-- empty attributes --&gt;</span>
<span class="nt">&lt;input</span> <span class="na">name=</span><span class="s">address</span> <span class="na">disabled</span><span class="nt">&gt;</span>
<span class="nt">&lt;input</span> <span class="na">name=</span><span class="s">address</span> <span class="na">disabled=</span><span class="s">""</span><span class="nt">&gt;</span>

<span class="c">&lt;!-- attributes with a value --&gt;</span>
<span class="nt">&lt;input</span> <span class="na">name=</span><span class="s">address</span> <span class="na">maxlength=</span><span class="s">200</span><span class="nt">&gt;</span>
<span class="nt">&lt;input</span> <span class="na">name=</span><span class="s">address</span> <span class="na">maxlength=</span><span class="s">'200'</span><span class="nt">&gt;</span>
<span class="nt">&lt;input</span> <span class="na">name=</span><span class="s">address</span> <span class="na">maxlength=</span><span class="s">"200"</span><span class="nt">&gt;</span>
</pre>
    <p>HTML user agents (e.g., Web browsers) then <a data-link-type="dfn" href="infrastructure.html#reparsed" id="ref-for-reparsed-1">parse</a> this markup, turning it into a DOM
  (Document Object Model) tree. A DOM tree is an in-memory representation of a document.</p>
    <p>DOM trees contain several kinds of nodes, in particular a <code class="idl"><a data-link-type="idl" href="https://www.w3.org/TR/dom/#documenttype">DocumentType</a></code> node, <code class="idl"><a data-link-type="idl" href="https://www.w3.org/TR/dom/#interface-element">Element</a></code> nodes, <code class="idl"><a data-link-type="idl" href="https://www.w3.org/TR/dom/#text">Text</a></code> nodes, <code class="idl"><a data-link-type="idl" href="https://www.w3.org/TR/dom/#comment">Comment</a></code> nodes, and in some cases <code class="idl"><a data-link-type="idl" href="https://www.w3.org/TR/dom/#processinginstruction">ProcessingInstruction</a></code> nodes.</p>
    <p>The <a href="introduction.html#intro-early-example">markup snippet at the top of this section</a> would be turned
  into the following DOM tree:</p>
    <ul class="domTree">
     <li class="t10">DOCTYPE: <code>html</code>
     </li><li class="t1">
       <code><a data-link-type="element" href="semantics.html#elementdef-html" id="ref-for-elementdef-html-1">html</a></code> 
      <ul>
       <li class="t1">
         <code><a data-link-type="element" href="document-metadata.html#elementdef-head" id="ref-for-elementdef-head-1">head</a></code> 
        <ul>
         <li class="t3"><code>#text</code>: <span>⏎␣␣</span>
         </li><li class="t1">
           <a data-link-type="element" href="document-metadata.html#elementdef-title" id="ref-for-elementdef-title-1"><code>title</code></a> 
          <ul>
           <li class="t3"><code>#text</code>: <span>Sample page</span>
          </li></ul>
         </li><li class="t3"><code>#text</code>: <span>⏎␣</span>
        </li></ul>
       </li><li class="t3"><code>#text</code>: <span>⏎␣</span>
       </li><li class="t1">
         <code><a data-link-type="element" href="sections.html#elementdef-body" id="ref-for-elementdef-body-3">body</a></code> 
        <ul>
         <li class="t3"><code>#text</code>: <span>⏎␣␣</span>
         </li><li class="t1">
           <code><a data-link-type="element" href="sections.html#elementdef-h1" id="ref-for-elementdef-h1-1">h1</a></code> 
          <ul>
           <li class="t3"><code>#text</code>: <span>Sample page</span>
          </li></ul>
         </li><li class="t3"><code>#text</code>: <span>⏎␣␣</span>
         </li><li class="t1">
           <code><a data-link-type="element" href="grouping-content.html#elementdef-p" id="ref-for-elementdef-p-1">p</a></code> 
          <ul>
           <li class="t3"><code>#text</code>: <span>This is a</span>
           </li><li class="t1">
             <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-a" id="ref-for-elementdef-a-2">a</a></code> <span class="t2"><code><a data-link-type="element-sub" href="links.html#element-attrdef-a-href" id="ref-for-element-attrdef-a-href-2">href</a></code>="<code>demo.html</code>"</span> 
            <ul>
             <li class="t3"><code>#text</code>: <span>simple</span>
            </li></ul>
           </li><li class="t3"><code>#text</code>: <span> sample.</span>
          </li></ul>
         </li><li class="t3"><code>#text</code>: <span>⏎␣␣</span>
         </li><li class="t8"><code>#comment</code>: <span> this is a comment </span>
         </li><li class="t3"><code>#text</code>: <span>⏎␣⏎</span>
        </li></ul>
      </li></ul>
    </li></ul>
    <p>The <a data-link-type="dfn" href="infrastructure.html#root-element" id="ref-for-root-element-1">root element</a> of this tree is the <code><a data-link-type="element" href="semantics.html#elementdef-html" id="ref-for-elementdef-html-2">html</a></code> element, which is the element always found at
  the root of HTML documents. It contains two elements, <code><a data-link-type="element" href="document-metadata.html#elementdef-head" id="ref-for-elementdef-head-2">head</a></code> and <code><a data-link-type="element" href="sections.html#elementdef-body" id="ref-for-elementdef-body-4">body</a></code>, as well as a <code class="idl"><a class="idl-code" data-link-type="interface" href="https://www.w3.org/TR/dom/#text">Text</a></code> node between them.</p>
    <p>There are many more <code class="idl"><a class="idl-code" data-link-type="interface" href="https://www.w3.org/TR/dom/#text">Text</a></code> nodes in the DOM tree than one would initially expect,
  because the source contains a number of spaces (represented here by "␣") and line breaks
  ("⏎") that all end up as <code class="idl"><a class="idl-code" data-link-type="interface" href="https://www.w3.org/TR/dom/#text">Text</a></code> nodes in the DOM. However, for historical
  reasons not all of the spaces and line breaks in the original markup appear in the DOM. In
  particular, all the whitespace before <code><a data-link-type="element" href="document-metadata.html#elementdef-head" id="ref-for-elementdef-head-3">head</a></code> start tag ends up being dropped silently, and all
  the whitespace after the <code><a data-link-type="element" href="sections.html#elementdef-body" id="ref-for-elementdef-body-5">body</a></code> end tag ends up placed at the end of the <code><a data-link-type="element" href="sections.html#elementdef-body" id="ref-for-elementdef-body-6">body</a></code>.</p>
    <p>The <code><a data-link-type="element" href="document-metadata.html#elementdef-head" id="ref-for-elementdef-head-4">head</a></code> element contains a <a data-link-type="element" href="document-metadata.html#elementdef-title" id="ref-for-elementdef-title-2"><code>title</code></a> element, which itself
  contains a <code class="idl"><a class="idl-code" data-link-type="interface" href="https://www.w3.org/TR/dom/#text">Text</a></code> node with the text "Sample page". Similarly, the <code><a data-link-type="element" href="sections.html#elementdef-body" id="ref-for-elementdef-body-7">body</a></code> element
  contains an <code><a data-link-type="element" href="sections.html#elementdef-h1" id="ref-for-elementdef-h1-2">h1</a></code> element, a <code><a data-link-type="element" href="grouping-content.html#elementdef-p" id="ref-for-elementdef-p-2">p</a></code> element, and a comment.</p>
    <hr>
    <p>This DOM tree can be manipulated from scripts in the page. Scripts (typically in JavaScript)
  are small programs that can be embedded using the <code><a data-link-type="element" href="semantics-scripting.html#elementdef-script" id="ref-for-elementdef-script-2">script</a></code> element or using <a data-link-type="dfn" href="webappapis.html#event-handler-content-event-handler-content-attribute" id="ref-for-event-handler-content-event-handler-content-attribute-1">event handler content attributes</a>. For example, here is a form with a script that sets the
  value of the form’s <code><a data-link-type="element" href="sec-forms.html#elementdef-output" id="ref-for-elementdef-output-1">output</a></code> element to say "Hello World"</p>
<pre class="highlight"><span class="nt">&lt;form</span> <span class="na">name=</span><span class="s">"main"</span><span class="nt">&gt;</span>
  Result: <span class="nt">&lt;output</span> <span class="na">name=</span><span class="s">"result"</span><span class="nt">&gt;&lt;/output&gt;</span>
  <span class="nt">&lt;script&gt;</span>
    <span class="nb">document</span><span class="p">.</span><span class="nx">forms</span><span class="p">.</span><span class="nx">main</span><span class="p">.</span><span class="nx">elements</span><span class="p">.</span><span class="nx">result</span><span class="p">.</span><span class="nx">value</span> <span class="o">=</span> <span class="s1">'Hello World'</span><span class="p">;</span>
  <span class="nt">&lt;/script&gt;</span>
<span class="nt">&lt;/form&gt;</span>
</pre>
    <p>Each element in the DOM tree is represented by an object, and these objects have APIs so that
  they can be manipulated. For instance, a link (e.g., the <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-a" id="ref-for-elementdef-a-3">a</a></code> element in the tree above) can have
  its "<code><a data-link-type="element-sub" href="links.html#element-attrdef-a-href" id="ref-for-element-attrdef-a-href-3">href</a></code>" attribute changed in several ways:</p>
<pre class="highlight"><span class="kd">var</span> <span class="nx">a</span> <span class="o">=</span> <span class="nb">document</span><span class="p">.</span><span class="nx">links</span><span class="p">[</span><span class="mi">0</span><span class="p">];</span> <span class="c1">// obtain the first link in the document</span>
<span class="nx">a</span><span class="p">.</span><span class="nx">href</span> <span class="o">=</span> <span class="s1">'sample.html'</span><span class="p">;</span> <span class="c1">// change the destination URL of the link</span>
<span class="nx">a</span><span class="p">.</span><span class="nx">protocol</span> <span class="o">=</span> <span class="s1">'https'</span><span class="p">;</span> <span class="c1">// change just the scheme part of the URL</span>
<span class="nx">a</span><span class="p">.</span><span class="nx">setAttribute</span><span class="p">(</span><span class="s1">'href'</span><span class="p">,</span> <span class="s1">'https://example.com/'</span><span class="p">);</span> <span class="c1">// change the content attribute directly</span>
</pre>
    <p>Since DOM trees are used as the way to represent HTML documents when they are processed and
  presented by implementations (especially interactive implementations like Web browsers), this
  specification is mostly phrased in terms of DOM trees, instead of the markup described above.</p>
    <hr>
    <p>HTML documents represent a media-independent description of interactive content. HTML documents
  might be rendered to a screen, or through a speech synthesizer, or on a braille display. To
  influence exactly how such rendering takes place, authors can use a styling language such as CSS.</p>
    <p>In the following example, the page has been made yellow-on-blue using CSS.</p>
<pre class="highlight"><span class="cp">&lt;!DOCTYPE html&gt;</span>
<span class="nt">&lt;html&gt;</span>
  <span class="nt">&lt;head&gt;</span>
    <span class="nt">&lt;title&gt;</span>Sample styled page<span class="nt">&lt;/title&gt;</span>
    <span class="nt">&lt;style&gt;</span>
      <span class="nt">body</span> <span class="p">{</span> <span class="k">background</span><span class="o">:</span> <span class="nb">navy</span><span class="p">;</span> <span class="k">color</span><span class="o">:</span> <span class="nb">yellow</span><span class="p">;</span> <span class="p">}</span>
    <span class="nt">&lt;/style&gt;</span>
  <span class="nt">&lt;/head&gt;</span>
  <span class="nt">&lt;body&gt;</span>
    <span class="nt">&lt;h1&gt;</span>Sample styled page<span class="nt">&lt;/h1&gt;</span>
    <span class="nt">&lt;p&gt;</span>This page is just a demo.<span class="nt">&lt;/p&gt;</span>
  <span class="nt">&lt;/body&gt;</span>
<span class="nt">&lt;/html&gt;</span>
</pre>
    <p>For more details on how to use HTML, authors are encouraged to consult tutorials and guides. Some
  of the examples included in this specification might also be of use, but the novice author is
  cautioned that this specification, by necessity, defines the language with a level of detail that
  might be difficult to understand at first.</p>
    <h4 class="heading settled" data-level="1.9.1" id="writing-secure-applications-with-html"><span class="secno">1.9.1. </span><span class="content">Writing secure applications with HTML</span><a class="self-link" href="introduction.html#writing-secure-applications-with-html"></a></h4>
    <p><em>This section is non-normative.</em></p>
    <p>When HTML is used to create interactive sites, care needs to be taken to avoid introducing
  vulnerabilities through which attackers can compromise the integrity of the site itself or of
  the site’s users.</p>
    <p>A comprehensive study of this matter is beyond the scope of this document, and authors are
  strongly encouraged to study the matter in more detail. However, this section attempts to provide
  a quick introduction to some common pitfalls in HTML application development.</p>
    <p>The security model of the Web is based on the concept of "origins", and correspondingly many of
  the potential attacks on the Web involve cross-origin actions. <a data-link-type="biblio" href="references.html#biblio-origin">[ORIGIN]</a></p>
    <dl>
     <dt data-md="">
      <p>Not validating user input</p>
     </dt><dt data-md="">
      <p>Cross-site scripting (XSS)</p>
     </dt><dt data-md="">
      <p>SQL injection</p>
     </dt><dd data-md="">
      <p>When accepting untrusted input, e.g., user-generated content such as text comments, values
  in URL parameters, messages from third-party sites, etc, it is imperative that the data
  be validated before use, and properly escaped when displayed. Failing to do this can
  allow a hostile user to perform a variety of attacks, ranging from the potentially
  benign, such as providing bogus user information like a negative age, to the serious,
  such as running scripts every time a user looks at a page that includes the information,
  potentially propagating the attack in the process, to the catastrophic, such as deleting
  all data in the server.</p>
      <p>When writing filters to validate user input, it is imperative that filters always be
  safelist-based, allowing known-safe constructs and disallowing all other input.
  Blocklist-based filters that disallow known-bad inputs and allow everything else are not
  secure, as not everything that is bad is yet known (for example, because it might be invented
  in the future).</p>
      <div class="example" id="example-3300faf0">
       <a class="self-link" href="introduction.html#example-3300faf0"></a> For example, suppose a page looked at its URL’s query string to determine what to display,
    and the site then redirected the user to that page to display a message, as in: 
<pre class="highlight"><span class="nt">&lt;ul&gt;</span>
  <span class="nt">&lt;li&gt;&lt;a</span> <span class="na">href=</span><span class="s">"message.cgi?say=Hello"</span><span class="nt">&gt;</span>Say Hello<span class="nt">&lt;/a&gt;</span>
  <span class="nt">&lt;li&gt;&lt;a</span> <span class="na">href=</span><span class="s">"message.cgi?say=Welcome"</span><span class="nt">&gt;</span>Say Welcome<span class="nt">&lt;/a&gt;</span>
  <span class="nt">&lt;li&gt;&lt;a</span> <span class="na">href=</span><span class="s">"message.cgi?say=Kittens"</span><span class="nt">&gt;</span>Say Kittens<span class="nt">&lt;/a&gt;</span>
<span class="nt">&lt;/ul&gt;</span>
</pre>
       <p>If the message was just displayed to the user without escaping, a hostile attacker could
    then craft a URL that contained a script element:</p>
<pre>https://example.com/message.cgi?say=%3Cscript%3Ealert%28%27Oh%20no%21%27%29%3C/script%3E</pre>
       <p>If the attacker then convinced a victim user to visit this page, a script of the
    attacker’s choosing would run on the page. Such a script could do any number of hostile
    actions, limited only by what the site offers: if the site is an e-commerce shop, for
    instance, such a script could cause the user to unknowingly make arbitrarily many
    unwanted purchases.</p>
       <p>This is called a cross-site scripting attack.</p>
      </div>
      <p>There are many constructs that can be used to try to trick a site into executing code. Here
  are some that authors are encouraged to consider when writing safelist filters:</p>
      <ul>
       <li data-md="">
        <p>When allowing harmless-seeming elements like <code><a data-link-type="element" href="semantics-embedded-content.html#elementdef-img" id="ref-for-elementdef-img-1">img</a></code>, exercise the principle of
least-privilege and limit the element’s attributes to only those that are needed (e.g., via
a safelist). If one allowed all attributes then an attacker could, for instance, use the <a class="idl-code" data-link-type="attribute" href="webappapis.html#dom-globaleventhandlers-onload" id="ref-for-dom-globaleventhandlers-onload-1"><code>onload</code></a> attribute to run arbitrary
script.</p>
       </li><li data-md="">
        <p>When allowing URLs to be provided (e.g., for links), the scheme of each URL also needs to be
explicitly safelisted, as there are many schemes that can be abused. The most prominent
example is "<code>javascript:</code>", but user agents can implement (and indeed, have
historically implemented) others.</p>
       </li><li data-md="">
        <p>Allowing a <code><a data-link-type="element" href="document-metadata.html#elementdef-base" id="ref-for-elementdef-base-1">base</a></code> element to be inserted means any <code><a data-link-type="element" href="semantics-scripting.html#elementdef-script" id="ref-for-elementdef-script-3">script</a></code> elements in the page with
relative links can be hijacked, and similarly that any form submissions can get redirected
to a hostile site.</p>
      </li></ul>
     </dd><dt data-md="">
      <p>Cross-site request forgery (CSRF)</p>
     </dt><dd data-md="">
      <p>If a site allows a user to make form submissions with user-specific side-effects, for example
  posting messages on a forum under the user’s name, making purchases, or applying for a
  passport, it is important to verify that the request was made by the user intentionally,
  rather than by another site tricking the user into making the request unknowingly.</p>
      <p>This problem exists because HTML forms can be submitted to other origins.</p>
      <p>Sites can prevent such attacks by populating forms with user-specific hidden tokens, or by
  checking <code>Origin</code> headers on all requests.</p>
     </dd><dt data-md="">
      <p>Clickjacking</p>
     </dt><dd data-md="">
      <p>A page that provides users with an interface to perform actions that the user might not
  wish to perform needs to be designed so as to avoid the possibility that users can be
  tricked into activating the interface.</p>
      <p>One way that a user could be so tricked is if a hostile site places the victim site in a
  small <code><a data-link-type="element" href="semantics-embedded-content.html#elementdef-iframe" id="ref-for-elementdef-iframe-1">iframe</a></code> and then convinces the user to click, for instance by having the user play
  a reaction game. Once the user is playing the game, the hostile site can quickly position
  the <code><a data-link-type="element" href="semantics-embedded-content.html#elementdef-iframe" id="ref-for-elementdef-iframe-2">iframe</a></code> under the mouse cursor just as the user is about to click, thus tricking the
  user into clicking the victim site’s interface.</p>
      <p>To avoid this, sites that do not expect to be used in frames are encouraged to only enable
  their interface if they detect that they are not in a frame (e.g., by comparing the <code class="idl"><a data-link-type="idl" href="browsers.html#window-window" id="ref-for-window-window-1">Window</a></code> object to the value of the <code class="idl"><a data-link-type="idl" href="browsers.html#dom-window-top" id="ref-for-dom-window-top-1">top</a></code> attribute).</p>
    </dd></dl>
    <h4 class="heading settled" data-level="1.9.2" id="common-pitfalls-to-avoid-when-using-the-scripting-apis"><span class="secno">1.9.2. </span><span class="content"> Common pitfalls to avoid when using the scripting APIs </span><a class="self-link" href="introduction.html#common-pitfalls-to-avoid-when-using-the-scripting-apis"></a></h4>
    <p><em>This section is non-normative.</em></p>
    <p>Scripts in HTML have "run-to-completion" semantics, meaning that the browser will generally run
  the script uninterrupted before doing anything else, such as firing further events or continuing
  to parse the document.</p>
    <p>On the other hand, parsing of HTML files happens <a data-link-type="dfn" href="infrastructure.html#in-parallel" id="ref-for-in-parallel-1">in parallel</a> and incrementally, meaning
  that the parser can pause at any point to let scripts run. This is generally a good thing, but it
  does mean that authors need to be careful to avoid hooking event handlers after the events could
  have possibly fired.</p>
    <p>There are two techniques for doing this reliably: use <a data-link-type="dfn" href="webappapis.html#event-handler-content-event-handler-content-attribute" id="ref-for-event-handler-content-event-handler-content-attribute-2">event handler content attributes</a>, or
  create the element and add the event handlers in the same script. The latter is safe because, as
  mentioned earlier, scripts are run to completion before further events can fire.</p>
    <div class="example" id="example-92cd6441">
     <a class="self-link" href="introduction.html#example-92cd6441"></a> One way this could manifest itself is with <code><a data-link-type="element" href="semantics-embedded-content.html#elementdef-img" id="ref-for-elementdef-img-2">img</a></code> elements and the <code>load</code> event.
    The event could fire as soon as the element has been parsed, especially if the image has already
    been cached (which is common). 
     <p>Here, the author uses the <a class="idl-code" data-link-type="attribute" href="webappapis.html#dom-globaleventhandlers-onload" id="ref-for-dom-globaleventhandlers-onload-2"><code>onload</code></a> handler
    on an <code><a data-link-type="element" href="semantics-embedded-content.html#elementdef-img" id="ref-for-elementdef-img-3">img</a></code> element to catch the <a class="idl-code" data-link-type="event" href="fullindex.html#eventdef-global-load" id="ref-for-eventdef-global-load-1"><code>load</code></a> event:</p>
<pre class="highlight"><span class="nt">&lt;img</span> <span class="na">src=</span><span class="s">"games.png"</span> <span class="na">alt=</span><span class="s">"Games"</span> <span class="na">onload=</span><span class="s">"gamesLogoHasLoaded(event)"</span><span class="nt">&gt;</span>
</pre>
     <p>If the element is being added by script, then so long as the event handlers are added in the
    same script, the event will still not be missed:</p>
<pre class="highlight"><span class="nt">&lt;script&gt;</span>
<span class="kd">var</span> <span class="nx">img</span> <span class="o">=</span> <span class="k">new</span> <span class="nx">Image</span><span class="p">();</span>
<span class="nx">img</span><span class="p">.</span><span class="nx">src</span> <span class="o">=</span> <span class="s1">'games.png'</span><span class="p">;</span>
<span class="nx">img</span><span class="p">.</span><span class="nx">alt</span> <span class="o">=</span> <span class="s1">'Games'</span><span class="p">;</span>
<span class="nx">img</span><span class="p">.</span><span class="nx">onload</span> <span class="o">=</span> <span class="nx">gamesLogoHasLoaded</span><span class="p">;</span>
<span class="c1">// img.addEventListener('load', gamesLogoHasLoaded, false); // would work also</span>
<span class="nt">&lt;/script&gt;</span>
</pre>
     <p>However, if the author first created the <code><a data-link-type="element" href="semantics-embedded-content.html#elementdef-img" id="ref-for-elementdef-img-4">img</a></code> element and then in a separate
    script added the event listeners, there’s a chance that the <code>load</code> event would be
    fired in between, leading it to be missed:</p>
<pre class="bad highlight"><span class="c">&lt;!-- Do not use this style, it has a race condition! --&gt;</span>
<span class="nt">&lt;img</span> <span class="na">id=</span><span class="s">"games"</span> <span class="na">src=</span><span class="s">"games.png"</span> <span class="na">alt=</span><span class="s">"Games"</span><span class="nt">&gt;</span>
<span class="c">&lt;!-- the 'load' event might fire here while the parser is taking a</span>
<span class="c">    break, in which case you will not see it! --&gt;</span>
<span class="nt">&lt;script&gt;</span>
<span class="kd">var</span> <span class="nx">img</span> <span class="o">=</span> <span class="nb">document</span><span class="p">.</span><span class="nx">getElementById</span><span class="p">(</span><span class="s1">'games'</span><span class="p">);</span>
<span class="nx">img</span><span class="p">.</span><span class="nx">onload</span> <span class="o">=</span> <span class="nx">gamesLogoHasLoaded</span><span class="p">;</span> <span class="c1">// might never fire!</span>
<span class="nt">&lt;/script&gt;</span>
</pre>
    </div>
    <h4 class="heading settled" data-level="1.9.3" id="how-to-catch-mistakes-when-writing-html-validators-and-conformance-checkers"><span class="secno">1.9.3. </span><span class="content"> How to catch mistakes when writing HTML: validators and conformance checkers </span><a class="self-link" href="introduction.html#how-to-catch-mistakes-when-writing-html-validators-and-conformance-checkers"></a></h4>
    <p><em>This section is non-normative.</em></p>
    <p>Authors are encouraged to make use of conformance checkers (also known as <i>validators</i>) to
  catch common mistakes. The W3C provides a number of online validation services, including the <a data-link-type="dfn" href="https://validator.w3.org/nu/#">Nu Markup Validation Service</a>.</p>
    <h3 class="heading settled" data-level="1.10" id="conformance-requirements-for-authors"><span class="secno">1.10. </span><span class="content">Conformance requirements for authors</span><a class="self-link" href="introduction.html#conformance-requirements-for-authors"></a></h3>
    <p><em>This section is non-normative.</em></p>
    <p>Unlike previous versions of the HTML specification, this specification defines in some detail the
  required processing for invalid documents as well as valid documents.</p>
    <p>However, even though the processing of invalid content is in most cases well-defined, conformance
  requirements for documents are still important: in practice, interoperability (the situation in
  which all implementations process particular content in a reliable and identical or equivalent
  way) is not the only goal of document conformance requirements. This section details some of the
  more common reasons for still distinguishing between a <a data-link-type="dfn" href="infrastructure.html#conforming-document" id="ref-for-conforming-document-1">conforming document</a> and one with errors.</p>
    <h4 class="heading settled" data-level="1.10.1" id="presentational-markup"><span class="secno">1.10.1. </span><span class="content">Presentational markup</span><a class="self-link" href="introduction.html#presentational-markup"></a></h4>
    <p><em>This section is non-normative.</em></p>
    <p>The majority of presentational features from previous versions of HTML are no longer allowed.
  Presentational markup in general has been found to have a number of problems:</p>
    <dl>
     <dt data-md="">
      <p>The use of presentational elements leads to poorer accessibility</p>
     </dt><dd data-md="">
      <p>While it is possible to use presentational markup in a way that provides users of assistive
  technologies (ATs) with an acceptable experience (e.g., using ARIA), doing so is significantly
  more difficult than doing so when using semantically-appropriate markup. Furthermore,
  presentational markup does not guarantee accessibility for users of non-AT, non-graphical user
  agents (such as text-mode browsers).</p>
      <p>Using media-independent markup, on the other hand, provides an easy way for documents to be
  authored in such a way that they are "accessible" for more users (e.g., users of text
  browsers).</p>
     </dd><dt data-md="">
      <p>Higher cost of maintenance</p>
     </dt><dd data-md="">
      <p>It is significantly easier to maintain a site written in such a way that the markup is
  style-independent. For example, changing the color of a site that uses <code>&lt;font&nbsp;color=""&gt;</code> throughout requires changes across the entire site,
  whereas a similar change to a site based on CSS can be done by changing a single file.</p>
     </dd><dt data-md="">
      <p>Larger document sizes</p>
     </dt><dd data-md="">
      <p>Presentational markup tends to be much more redundant, and thus results in larger document
  sizes.</p>
    </dd></dl>
    <p>For those reasons, presentational markup has been removed from HTML in this version. This
  change should not come as a surprise; HTML 4.0 deprecated presentational markup many years ago
  and provided a mode (HTML Transitional) to help authors move away from presentational markup;
  later, XHTML 1.1 went further and obsoleted those features altogether.</p>
    <p>The only remaining presentational markup features in HTML are the <code><a data-link-type="element-sub" href="dom.html#element-attrdef-global-style" id="ref-for-element-attrdef-global-style-1">style</a></code> attribute
  and the <code><a data-link-type="element" href="document-metadata.html#elementdef-style" id="ref-for-elementdef-style-1">style</a></code> element. Use of the <code><a data-link-type="element-sub" href="dom.html#element-attrdef-global-style" id="ref-for-element-attrdef-global-style-2">style</a></code> attribute is somewhat discouraged in
  production environments, but it can be useful for rapid prototyping (where its rules can be
  directly moved into a separate style sheet later) and for providing specific styles in unusual
  cases where a separate style sheet would be inconvenient. Similarly, the <code><a data-link-type="element" href="document-metadata.html#elementdef-style" id="ref-for-elementdef-style-2">style</a></code> element can
  be useful for grouping or for page-specific styles, but in general an external style sheet
  is likely to be more convenient when the styles apply to multiple pages.</p>
    <p>It is also worth noting that some elements that were previously presentational have been
  redefined in this specification to be media-independent: <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-b" id="ref-for-elementdef-b-1">b</a></code>, <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-i" id="ref-for-elementdef-i-1">i</a></code>, <code><a data-link-type="element" href="grouping-content.html#elementdef-hr" id="ref-for-elementdef-hr-1">hr</a></code>, <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-s" id="ref-for-elementdef-s-1">s</a></code>, <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-small" id="ref-for-elementdef-small-1">small</a></code>, and <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-u" id="ref-for-elementdef-u-1">u</a></code>.</p>
    <h4 class="heading settled" data-level="1.10.2" id="syntax-errors"><span class="secno">1.10.2. </span><span class="content">Syntax errors</span><a class="self-link" href="introduction.html#syntax-errors"></a></h4>
    <p><em>This section is non-normative.</em></p>
    <p>The syntax of HTML is constrained to avoid a wide variety of problems.</p>
    <dl>
     <dt data-md="">
      <p>Unintuitive error-handling behavior</p>
     </dt><dd data-md="">
      <p>Certain invalid syntax constructs, when parsed, result in DOM trees that are highly
  unintuitive.</p>
      <div class="example" id="example-0dccb7e3">
       <a class="self-link" href="introduction.html#example-0dccb7e3"></a> For example, the following markup fragment results in a DOM with an <code><a data-link-type="element" href="grouping-content.html#elementdef-hr" id="ref-for-elementdef-hr-2">hr</a></code> element that is
    an <em>earlier</em> sibling of the corresponding <code><a data-link-type="element" href="tabular-data.html#elementdef-table" id="ref-for-elementdef-table-1">table</a></code> element: 
<pre class="bad highlight"><span class="nt">&lt;table&gt;&lt;hr&gt;</span>...
</pre>
      </div>
     </dd><dt data-md="">
      <p>Errors with optional error recovery</p>
     </dt><dd data-md="">
      <p>To allow user agents to be used in controlled environments without having to implement the more
  bizarre and convoluted error handling rules, user agents are permitted to fail whenever
  encountering a <a data-link-type="dfn" href="syntax.html#parser-parse-error" id="ref-for-parser-parse-error-1">parse error</a>.</p>
     </dd><dt data-md="">
      <p>Errors where the error-handling behavior is not compatible with streaming user agents</p>
     </dt><dd data-md="">
      <p>Some error-handling behavior, such as the behavior for the <code>&lt;table&gt;&lt;hr&gt;...</code> example mentioned above, are incompatible with streaming user agents (user agents that process
  HTML files in one pass, without storing state). To avoid interoperability problems with such
  user agents, any syntax resulting in such behavior is considered invalid.</p>
     </dd><dt data-md="">
      <p>Errors that can result in infoset coercion</p>
     </dt><dd data-md="">
      <p>When a user agent based on XML is connected to an HTML parser, it is possible that certain
  invariants that XML enforces, such as comments never containing two consecutive hyphens, will
  be violated by an HTML file. Handling this can require that the parser coerce the HTML DOM
  into an XML-compatible infoset. Most syntax constructs that require such handling are
  considered invalid.</p>
     </dd><dt data-md="">
      <p>Errors that result in disproportionately poor performance</p>
     </dt><dd data-md="">
      <p>Certain syntax constructs can result in disproportionately poor performance. To discourage the
  use of such constructs, they are typically made non-conforming.</p>
      <div class="example" id="example-ccb699bd">
       <a class="self-link" href="introduction.html#example-ccb699bd"></a> For example, the following markup results in poor performance, since all the unclosed <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-i" id="ref-for-elementdef-i-2">i</a></code> elements have to be reconstructed in each paragraph, resulting in progressively more
    elements in each paragraph: 
<pre class="bad highlight"><span class="nt">&lt;p&gt;&lt;i&gt;</span>He dreamt.
<span class="nt">&lt;p&gt;&lt;i&gt;</span>He dreamt that he ate breakfast.
<span class="nt">&lt;p&gt;&lt;i&gt;</span>Then lunch.
<span class="nt">&lt;p&gt;&lt;i&gt;</span>And finally dinner.
</pre>
       <p>The resulting DOM for this fragment would be:</p>
       <ul class="domTree">
        <li class="t1">
          <code><a data-link-type="element" href="grouping-content.html#elementdef-p" id="ref-for-elementdef-p-3">p</a></code> 
         <ul>
          <li class="t1">
            <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-i" id="ref-for-elementdef-i-3">i</a></code> 
           <ul>
            <li class="t3"><code>#text</code>: <span>He dreamt.</span>
           </li></ul>
         </li></ul>
        </li><li class="t1">
          <code><a data-link-type="element" href="grouping-content.html#elementdef-p" id="ref-for-elementdef-p-4">p</a></code> 
         <ul>
          <li class="t1">
            <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-i" id="ref-for-elementdef-i-4">i</a></code> 
           <ul>
            <li class="t1">
              <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-i" id="ref-for-elementdef-i-5">i</a></code> 
             <ul>
              <li class="t3"><code>#text</code>: <span>He dreamt that he ate breakfast.</span>
             </li></ul>
           </li></ul>
         </li></ul>
        </li><li class="t1">
          <code><a data-link-type="element" href="grouping-content.html#elementdef-p" id="ref-for-elementdef-p-5">p</a></code> 
         <ul>
          <li class="t1">
            <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-i" id="ref-for-elementdef-i-6">i</a></code> 
           <ul>
            <li class="t1">
              <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-i" id="ref-for-elementdef-i-7">i</a></code> 
             <ul>
              <li class="t1">
                <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-i" id="ref-for-elementdef-i-8">i</a></code> 
               <ul>
                <li class="t3"><code>#text</code>: <span>Then lunch.</span>
               </li></ul>
             </li></ul>
           </li></ul>
         </li></ul>
        </li><li class="t1">
          <code><a data-link-type="element" href="grouping-content.html#elementdef-p" id="ref-for-elementdef-p-6">p</a></code> 
         <ul>
          <li class="t1">
            <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-i" id="ref-for-elementdef-i-9">i</a></code> 
           <ul>
            <li class="t1">
              <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-i" id="ref-for-elementdef-i-10">i</a></code> 
             <ul>
              <li class="t1">
                <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-i" id="ref-for-elementdef-i-11">i</a></code> 
               <ul>
                <li class="t1">
                  <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-i" id="ref-for-elementdef-i-12">i</a></code> 
                 <ul>
                  <li class="t3"><code>#text</code>: <span>And finally dinner.</span>
                 </li></ul>
               </li></ul>
             </li></ul>
           </li></ul>
         </li></ul>
       </li></ul>
      </div>
     </dd><dt data-md="">
      <p>Errors involving fragile syntax constructs</p>
     </dt><dd data-md="">
      <p>There are syntax constructs that, for historical reasons, are relatively fragile. To help
  reduce the number of users who accidentally run into such problems, they are made
  non-conforming.</p>
      <div class="example" id="example-4c330288">
       <a class="self-link" href="introduction.html#example-4c330288"></a> For example, the parsing of certain named character references in attributes happens even
    with the closing semicolon being omitted. It is safe to include an ampersand followed by
    letters that do not form a named character reference, but if the letters are changed to a
    string that <em>does</em> form a named character reference, they will be interpreted as that
    character instead. 
       <p>In this fragment, the attribute’s value is "<code>?bill&amp;ted</code>":</p>
<pre class="bad highlight"><span class="nt">&lt;a</span> <span class="na">href=</span><span class="s">"?bill&amp;ted"</span><span class="nt">&gt;</span>Bill and Ted<span class="nt">&lt;/a&gt;</span>
</pre>
       <p>In the following fragment, however, the attribute’s value is actually
      "<code>?art©</code>", <em>not</em> the intended "<code>?art&amp;copy</code>",
      because even without the final semicolon, "<code>&amp;copy</code>" is handled the same
      as "<code>&amp;copy;</code>" and thus gets interpreted as "<code>©</code>":</p>
<pre class="bad highlight"><span class="nt">&lt;a</span> <span class="na">href=</span><span class="s">"?art&amp;copy"</span><span class="nt">&gt;</span>Art and Copy<span class="nt">&lt;/a&gt;</span>
</pre>
       <p>To avoid this problem, all named character references are required to end with a
      semicolon, and uses of named character references without a semicolon are flagged as
      errors.</p>
       <p>Thus, the correct way to express the above cases is as follows:</p>
<pre class="highlight"><span class="nt">&lt;a</span> <span class="na">href=</span><span class="s">"?bill&amp;ted"</span><span class="nt">&gt;</span>Bill and Ted<span class="nt">&lt;/a&gt;</span> <span class="c">&lt;!-- &amp;ted is ok, since it’s not a named character reference --&gt;</span>
</pre>
<pre class="highlight"><span class="nt">&lt;a</span> <span class="na">href=</span><span class="s">"?art&amp;amp;copy"</span><span class="nt">&gt;</span>Art and Copy<span class="nt">&lt;/a&gt;</span> <span class="c">&lt;!-- the &amp; has to be escaped, since &amp;copy is a named character reference --&gt;</span>
</pre>
      </div>
     </dd><dt data-md="">
      <p>Errors involving known interoperability problems in legacy user agents</p>
     </dt><dd data-md="">
      <p>Certain syntax constructs are known to cause especially subtle or serious problems in legacy
  user agents, and are therefore marked as non-conforming to help authors avoid them.</p>
      <p class="example" id="example-75966b06"><a class="self-link" href="introduction.html#example-75966b06"></a> For example, this is why the U+0060 GRAVE ACCENT character (`) is not allowed in unquoted
    attributes. In certain legacy user agents, it is sometimes treated as a quote character. </p>
      <p class="example" id="example-767451ed"><a class="self-link" href="introduction.html#example-767451ed"></a> Another example of this is the DOCTYPE, which is required to trigger <a data-link-type="dfn" href="infrastructure.html#no-quirks-mode" id="ref-for-no-quirks-mode-1">no-quirks mode</a>,
    because the behavior of legacy user agents in <a data-link-type="dfn" href="infrastructure.html#quirks-mode" id="ref-for-quirks-mode-1">quirks mode</a> is often largely
    undocumented. </p>
     </dd><dt data-md="">
      <p>Errors that risk exposing authors to security attacks</p>
     </dt><dd data-md="">
      <p>Certain restrictions exist purely to avoid known security problems.</p>
      <p class="example" id="example-7147ccfb"><a class="self-link" href="introduction.html#example-7147ccfb"></a> For example, the restriction on using UTF-7 exists purely to avoid authors falling prey to a
    known cross-site-scripting attack using UTF-7. <a data-link-type="biblio" href="references.html#biblio-rfc2152">[RFC2152]</a> </p>
     </dd><dt data-md="">
      <p>Cases where the author’s intent is unclear</p>
     </dt><dd data-md="">
      <p>Markup where the author’s intent is very unclear is often made non-conforming. Correcting these
  errors early makes later maintenance easier.</p>
      <p class="example" id="example-f40bc4c0"><a class="self-link" href="introduction.html#example-f40bc4c0"></a> For example, it is unclear whether the author intended the following to be an <code><a data-link-type="element" href="sections.html#elementdef-h1" id="ref-for-elementdef-h1-3">h1</a></code> heading
    or an <code><a data-link-type="element" href="sections.html#elementdef-h2" id="ref-for-elementdef-h2-1">h2</a></code> heading:</p>
<pre class="bad highlight"><span class="nt">&lt;h2&gt;</span>Contact details<span class="nt">&lt;/h1&gt;</span>
</pre>
     </dd><dt data-md="">
      <p>Cases that are likely to be typos</p>
     </dt><dd data-md="">
      <p>When a user makes a simple typo, it is helpful if the error can be caught early, as this can
  save the author a lot of debugging time. This specification therefore usually considers it
  an error to use element names, attribute names, and so forth, that do not match the names
  defined in this specification.</p>
      <p class="example" id="example-01d3376c"><a class="self-link" href="introduction.html#example-01d3376c"></a> For example, if the author typed <code>&lt;capton&gt;</code> instead of <code>&lt;caption&gt;</code>, this would be flagged as an error and the author could correct
    the typo immediately. </p>
     </dd><dt data-md="">
      <p>Errors that could interfere with new syntax in the future</p>
     </dt><dd data-md="">
      <p>In order to allow the language syntax to be extended in the future, certain otherwise
  harmless features are disallowed.</p>
      <p class="example" id="example-f720dd66"><a class="self-link" href="introduction.html#example-f720dd66"></a> For example, attributes in end tags are currently invalid and ignored. A future change to
    the language may make use of this syntax feature and can do so without conflicting with
    already-deployed (and valid!) content. </p>
    </dd></dl>
    <p>Some authors find it helpful to be in the practice of always quoting all attributes and always
  including all optional tags, preferring the consistency derived from such custom over the minor
  benefits of terseness afforded by making use of the flexibility of the HTML syntax. To aid such
  authors, conformance checkers can provide modes of operation wherein such conventions are
  enforced.</p>
    <h4 class="heading settled" data-level="1.10.3" id="restrictions-on-content-models-and-on-attribute-values"><span class="secno">1.10.3. </span><span class="content"> Restrictions on content models and on attribute values </span><a class="self-link" href="introduction.html#restrictions-on-content-models-and-on-attribute-values"></a></h4>
    <p><em>This section is non-normative.</em></p>
    <p>Beyond the syntax of the language, this specification also places restrictions on how elements
  and attributes can be specified. These restrictions are present for similar reasons:</p>
    <dl>
     <dt data-md="">
      <p>Errors involving content with dubious semantics</p>
     </dt><dd data-md="">
      <p>To avoid misuse of elements with defined meanings, content models are defined that restrict
  how elements can be nested when such nestings would be of dubious value.</p>
      <p class="example" id="example-d984ebcc"><a class="self-link" href="introduction.html#example-d984ebcc"></a> For example, this specification disallows nesting a <code><a data-link-type="element" href="sections.html#elementdef-section" id="ref-for-elementdef-section-1">section</a></code> element inside
    a <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-kbd" id="ref-for-elementdef-kbd-1">kbd</a></code> element, since it is highly unlikely for an author to indicate that
    an entire section should be keyed in. </p>
     </dd><dt data-md="">
      <p>Errors that involve a conflict in expressed semantics</p>
     </dt><dd data-md="">
      <p>Similarly, to draw the author’s attention to mistakes in the use of elements, clear
  contradictions in the semantics expressed are also considered conformance errors.</p>
      <div class="example" id="example-2e4388b0">
       <a class="self-link" href="introduction.html#example-2e4388b0"></a> In the fragments below, for example, the semantics are nonsensical: a separator cannot
    simultaneously be a cell, nor can a radio button be a progress bar. 
<pre class="bad highlight"><span class="nt">&lt;hr</span> <span class="na">role=</span><span class="s">"cell"</span><span class="nt">&gt;</span>
</pre>
<pre class="bad highlight"><span class="nt">&lt;input</span> <span class="na">type=</span><span class="s">radio</span> <span class="na">role=</span><span class="s">progressbar</span><span class="nt">&gt;</span>
</pre>
      </div>
      <p class="example" id="example-2eeb2419"><a class="self-link" href="introduction.html#example-2eeb2419"></a> Another example is the restrictions on the content models of the <code><a data-link-type="element" href="grouping-content.html#elementdef-ul" id="ref-for-elementdef-ul-1">ul</a></code> element, which only
    allows <code><a data-link-type="element" href="grouping-content.html#elementdef-li" id="ref-for-elementdef-li-1">li</a></code> element children. Lists by definition consist just of zero or more list items,
    so if a <code><a data-link-type="element" href="grouping-content.html#elementdef-ul" id="ref-for-elementdef-ul-2">ul</a></code> element contains something other than an <code><a data-link-type="element" href="grouping-content.html#elementdef-li" id="ref-for-elementdef-li-2">li</a></code> element, it’s not clear what
    was meant. </p>
     </dd><dt data-md="">
      <p>Cases where the default styles are likely to lead to confusion</p>
     </dt><dd data-md="">
      <p>Certain elements have default styles or behaviors that make certain combinations likely to
  lead to confusion. Where these have equivalent alternatives without this problem, the
  confusing combinations are disallowed.</p>
      <p class="example" id="example-fee18c64"><a class="self-link" href="introduction.html#example-fee18c64"></a> For example, <code><a data-link-type="element" href="grouping-content.html#elementdef-div" id="ref-for-elementdef-div-1">div</a></code> elements are rendered as block boxes, and <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-span" id="ref-for-elementdef-span-1">span</a></code> elements as inline
    boxes. Putting a block box in an inline box is unnecessarily confusing; since either nesting
    just <code><a data-link-type="element" href="grouping-content.html#elementdef-div" id="ref-for-elementdef-div-2">div</a></code> elements, or nesting just <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-span" id="ref-for-elementdef-span-2">span</a></code> elements, or nesting <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-span" id="ref-for-elementdef-span-3">span</a></code> elements
    inside <code><a data-link-type="element" href="grouping-content.html#elementdef-div" id="ref-for-elementdef-div-3">div</a></code> elements all serve the same purpose as nesting a <code><a data-link-type="element" href="grouping-content.html#elementdef-div" id="ref-for-elementdef-div-4">div</a></code> element in a <code><a data-link-type="element" href="textlevel-semantics.html#elementdef-span" id="ref-for-elementdef-span-4">span</a></code> element, but only the latter involves a block box in an inline box, the latter
    combination is disallowed. </p>
      <p class="example" id="example-940601d1"><a class="self-link" href="introduction.html#example-940601d1"></a> Another example would be the way <a data-link-type="dfn" href="dom.html#interactive-content" id="ref-for-interactive-content-1">interactive content</a> cannot be nested. For example, a <code><a data-link-type="element" href="sec-forms.html#elementdef-button" id="ref-for-elementdef-button-1">button</a></code> element cannot contain a <code><a data-link-type="element" href="sec-forms.html#elementdef-textarea" id="ref-for-elementdef-textarea-1">textarea</a></code> element. This is because the default
    behavior of such nesting interactive elements would be highly confusing to users. Instead of
    nesting these elements, they can be placed side by side. </p>
     </dd><dt data-md="">
      <p>Errors that indicate a likely misunderstanding of the specification</p>
     </dt><dd data-md="">
      <p>Sometimes, something is disallowed because allowing it would likely cause author confusion.</p>
      <p class="example" id="example-a37f5e13"><a class="self-link" href="introduction.html#example-a37f5e13"></a> For example, setting the <code><a data-link-type="element-sub" href="sec-forms.html#element-attrdef-disabledformelements-disabled" id="ref-for-element-attrdef-disabledformelements-disabled-1">disabled</a></code> attribute to the value
    "<code>false</code>" is disallowed, because despite the appearance of meaning that the
    element is enabled, it in fact means that the element is <em>disabled</em> (what matters for
    implementations is the presence of the attribute, not its value). </p>
     </dd><dt data-md="">
      <p>Errors involving limits that have been imposed merely to simplify the language</p>
     </dt><dd data-md="">
      <p>Some conformance errors simplify the language that authors need to learn.</p>
      <p class="example" id="example-fd0bf585"><a class="self-link" href="introduction.html#example-fd0bf585"></a> For example, the <code><a data-link-type="element" href="semantics-embedded-content.html#elementdef-area" id="ref-for-elementdef-area-1">area</a></code> element’s <code><a data-link-type="element-sub" href="semantics-embedded-content.html#element-attrdef-area-shape" id="ref-for-element-attrdef-area-shape-1">shape</a></code> attribute, despite accepting both <a data-link-type="attr-value" href="semantics-embedded-content.html#attr-valuedef-area-shape-circ" id="ref-for-attr-valuedef-area-shape-circ-1"><code>circ</code></a> and <a data-link-type="attr-value" href="semantics-embedded-content.html#attr-valuedef-area-shape-circle" id="ref-for-attr-valuedef-area-shape-circle-1"><code>circle</code></a> values in practice as synonyms,
    disallows the use of the <a data-link-type="attr-value" href="semantics-embedded-content.html#attr-valuedef-area-shape-circ" id="ref-for-attr-valuedef-area-shape-circ-2"><code>circ</code></a> value, so as
    to simplify tutorials and other learning aids. There would be no benefit to allowing both,
    but it would cause extra confusion when teaching the language. </p>
     </dd><dt data-md="">
      <p>Errors that involve peculiarities of the parser</p>
     </dt><dd data-md="">
      <p>Certain elements are parsed in somewhat eccentric ways (typically for historical reasons),
  and their content model restrictions are intended to avoid exposing the author to these
  issues.</p>
      <div class="example" id="example-6c4dba70">
       <a class="self-link" href="introduction.html#example-6c4dba70"></a> For example, a <code><a data-link-type="element" href="sec-forms.html#elementdef-form" id="ref-for-elementdef-form-1">form</a></code> element isn’t allowed inside <a data-link-type="dfn" href="dom.html#phrasing-content" id="ref-for-phrasing-content-1">phrasing content</a>, because when
    parsed as HTML, a <code><a data-link-type="element" href="sec-forms.html#elementdef-form" id="ref-for-elementdef-form-2">form</a></code> element’s start tag will imply a <code><a data-link-type="element" href="grouping-content.html#elementdef-p" id="ref-for-elementdef-p-7">p</a></code> element’s end tag. Thus,
    the following markup results in two <a data-link-type="dfn" href="dom.html#paragraph" id="ref-for-paragraph-1">paragraphs</a>, not one: 
<pre class="highlight"><span class="nt">&lt;p&gt;</span>Welcome. <span class="nt">&lt;form&gt;&lt;label&gt;</span>Name:<span class="nt">&lt;/label&gt;</span> <span class="nt">&lt;input&gt;&lt;/form&gt;</span>
</pre>
       <p>It is parsed exactly like the following:</p>
<pre class="highlight"><span class="nt">&lt;p&gt;</span>Welcome. <span class="nt">&lt;/p&gt;&lt;form&gt;&lt;label&gt;</span>Name:<span class="nt">&lt;/label&gt;</span> <span class="nt">&lt;input&gt;&lt;/form&gt;</span>
</pre>
      </div>
     </dd><dt data-md="">
      <p>Errors that would likely result in scripts failing in hard-to-debug ways</p>
     </dt><dd data-md="">
      <p>Some errors are intended to help prevent script problems that would be hard to debug.</p>
      <p class="example" id="example-4231d48e"><a class="self-link" href="introduction.html#example-4231d48e"></a> This is why, for instance, it is non-conforming to have two <code><a data-link-type="element-sub" href="dom.html#element-attrdef-global-id" id="ref-for-element-attrdef-global-id-1">id</a></code> attributes with
    the same value. Duplicate IDs lead to the wrong element being selected, with sometimes
    disastrous effects whose cause is hard to determine. </p>
     </dd><dt data-md="">
      <p>Errors that waste authoring time</p>
     </dt><dd data-md="">
      <p>Some constructs are disallowed because historically they have been the cause of a lot of
  wasted authoring time, and by encouraging authors to avoid making them, authors can save time
  in future efforts.</p>
      <p class="example" id="example-e37e480c"><a class="self-link" href="introduction.html#example-e37e480c"></a> For example, a <code><a data-link-type="element" href="semantics-scripting.html#elementdef-script" id="ref-for-elementdef-script-4">script</a></code> element’s <code><a data-link-type="element-sub" href="semantics-scripting.html#element-attrdef-script-src" id="ref-for-element-attrdef-script-src-1">src</a></code> attribute causes the element’s contents
    to be ignored. However, this isn’t obvious, especially if the element’s contents appear to
    be executable script — which can lead to authors spending a lot of time trying to
    debug the inline script without realizing that it is not executing. To reduce this problem,
    this specification makes it non-conforming to have executable script in a <code><a data-link-type="element" href="semantics-scripting.html#elementdef-script" id="ref-for-elementdef-script-5">script</a></code> element
    when the <code><a data-link-type="element-sub" href="semantics-scripting.html#element-attrdef-script-src" id="ref-for-element-attrdef-script-src-2">src</a></code> attribute is present. This means that authors who are validating
    their documents are less likely to waste time with this kind of mistake. </p>
     </dd><dt data-md="">
      <p>Errors that involve areas that affect authors migrating to and from XHTML</p>
     </dt><dd data-md="">
      <p>Some authors like to write files that can be interpreted as both XML and HTML with similar
  results. Though this practice is discouraged in general due to the myriad of subtle
  complications involved (especially when involving scripting, styling, or any kind of automated
  serialization), this specification has a few restrictions intended to at least somewhat
  mitigate the difficulties. This makes it easier for authors to use this as a transitionary
  step when migrating between HTML and XHTML.</p>
      <p class="example" id="example-cfd6bac9"><a class="self-link" href="introduction.html#example-cfd6bac9"></a> For example, there are somewhat complicated rules surrounding the <code><a data-link-type="element-sub" href="dom.html#element-attrdef-global-lang" id="ref-for-element-attrdef-global-lang-1">lang</a></code> and <code><a data-link-type="element-sub" href="dom.html#element-attrdef-xml-lang" id="ref-for-element-attrdef-xml-lang-1">xml:lang</a></code> attributes intended to keep the two synchronized. </p>
      <p class="example" id="example-fb89bc4a"><a class="self-link" href="introduction.html#example-fb89bc4a"></a> Another example would be the restrictions on the values of <code>xmlns</code> attributes
    in the HTML serialization, which are intended to ensure that elements in conforming
    documents end up in the same namespaces whether processed as HTML or XML. </p>
     </dd><dt data-md="">
      <p>Errors that involve areas reserved for future expansion</p>
     </dt><dd data-md="">
      <p>As with the restrictions on the syntax intended to allow for new syntax in future revisions
  of the language, some restrictions on the content models of elements and values of attributes
  are intended to allow for future expansion of the HTML vocabulary.</p>
      <p class="example" id="example-29868b5a"><a class="self-link" href="introduction.html#example-29868b5a"></a> For example, limiting the values of the <code><a data-link-type="element-sub" href="links.html#element-attrdef-a-target" id="ref-for-element-attrdef-a-target-1">target</a></code> attribute that start with an
    U+005F LOW LINE character (_) to only specific predefined values allows new predefined
    values to be introduced at a future time without conflicting with author-defined values. </p>
     </dd><dt data-md="">
      <p>Errors that indicate a mis-use of other specifications</p>
     </dt><dd data-md="">
      <p>Certain restrictions are intended to support the restrictions made by other specifications.</p>
      <p class="example" id="example-153135f0"><a class="self-link" href="introduction.html#example-153135f0"></a> For example, requiring that attributes that take media query lists use only <em>valid</em> media query lists reinforces the importance of following the conformance rules of that
    specification. </p>
    </dd></dl>
    <h3 class="heading settled" data-level="1.11" id="suggested-reading"><span class="secno">1.11. </span><span class="content">Suggested reading</span><a class="self-link" href="introduction.html#suggested-reading"></a></h3>
    <p><em>This section is non-normative.</em></p>
    <p>The following documents might be of interest to readers of this specification.</p>
    <dl>
     <dt data-md="">
      <p><cite>Character Model for the World Wide Web 1.0: Fundamentals</cite> <a data-link-type="biblio" href="references.html#biblio-charmod">[CHARMOD]</a></p>
     </dt><dd data-md="">
      <blockquote> This Architectural Specification provides authors of specifications, software developers,
    and content developers with a common reference for interoperable text manipulation on the
    World Wide Web, building on the Universal Character Set, defined jointly by the Unicode
    Standard and ISO/IEC 10646. Topics addressed include use of the terms "character",
    "encoding" and "string", a reference processing model, choice and
    identification of character encodings, character escaping, and string indexing. </blockquote>
     </dd><dt data-md="">
      <p><cite>Unicode Security Considerations</cite> <a data-link-type="biblio" href="references.html#biblio-unicode-security">[UNICODE-SECURITY]</a></p>
     </dt><dd data-md="">
      <blockquote> Because Unicode contains such a large number of characters and incorporates the varied
    writing systems of the world, incorrect usage can expose programs or systems to possible
    security attacks. This is especially important as more and more products are
    internationalized. This document describes some of the security considerations that
    programmers, system analysts, standards developers, and users should take into account, and
    provides specific recommendations to reduce the risk of problems. </blockquote>
     </dd><dt data-md="">
      <p><cite>Web Content Accessibility Guidelines (WCAG) 2.0</cite> <a data-link-type="biblio" href="references.html#biblio-wcag20">[WCAG20]</a></p>
     </dt><dd data-md="">
      <blockquote> Web Content Accessibility Guidelines (WCAG) 2.0 covers a wide range of recommendations for
    making Web content more accessible. Following these guidelines will make content accessible
    to a wider range of people with disabilities, including blindness and low vision, deafness
    and hearing loss, learning disabilities, cognitive limitations, limited movement, speech
    disabilities, photosensitivity and combinations of these. Following these guidelines will
    also often make your Web content more usable to users in general. </blockquote>
     </dd><dt data-md="">
      <p><cite>Authoring Tool Accessibility Guidelines (ATAG) 2.0</cite> <a data-link-type="biblio" href="references.html#biblio-atag20">[ATAG20]</a></p>
     </dt><dd data-md="">
      <blockquote> This specification provides guidelines for designing Web content authoring tools that are
    more accessible for people with disabilities. An authoring tool that conforms to these
    guidelines will promote accessibility by providing an accessible user interface to authors
    with disabilities as well as by enabling, supporting, and promoting the production of
    accessible Web content by all authors. </blockquote>
     </dd><dt data-md="">
      <p><cite>User Agent Accessibility Guidelines (UAAG) 2.0</cite> <a data-link-type="biblio" href="references.html#biblio-uaag20">[UAAG20]</a></p>
     </dt><dd data-md="">
      <blockquote> This document provides guidelines for designing user agents that lower barriers to Web
    accessibility for people with disabilities. User agents include browsers and other types of
    software that retrieve and render Web content. A user agent that conforms to these
    guidelines will promote accessibility through its own user interface and through other
    internal facilities, including its ability to communicate with other technologies
    (especially assistive technologies). Furthermore, all users, not just users with
    disabilities, should find conforming user agents to be more usable. </blockquote>
     </dd><dt data-md="">
      <p><cite>Polyglot Markup: HTML-Compatible XHTML Documents</cite> <a data-link-type="biblio" href="references.html#biblio-html-polyglot">[HTML-POLYGLOT]</a></p>
     </dt><dd data-md="">
      <blockquote> A document that uses polyglot markup is a document that is a stream of bytes that parses
    into identical document trees (with the exception of the xmlns attribute on the root
    element) when processed as HTML and when processed as XML. Polyglot markup that meets a
    well defined set of constraints is interpreted as compatible, regardless of whether they are
    processed as HTML or as XHTML, per the HTML specification. Polyglot markup uses a specific
    DOCTYPE, namespace declarations, and a specific case — normally lower case but
    occasionally camel case — for element and attribute names. Polyglot markup uses lower
    case for certain attribute values. Further constraints include those on empty elements,
    named entity references, and the use of scripts and style. </blockquote>
     </dd><dt data-md="">
      <p><cite>HTML Accessibility APIs Mappings 1.0</cite> <a data-link-type="biblio" href="references.html#biblio-html-aam-10">[HTML-AAM-1.0]</a></p>
     </dt><dd data-md="">
      <blockquote> Defines how user agents map HTML 5.1 elements and attributes to platform accessibility APIs.
    Documenting these mappings promotes interoperable exposure of roles, states, properties, and
    events implemented by accessibility APIs and helps to ensure that this information appears
    in a manner consistent with author intent. </blockquote>
    </dd></dl>
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